All aspects of cardiovascular function are regulated by receptors of the seven transmembrane receptor (7TMR or GPCR) families, and they are the commonest target of therapeutic drugs. A universal mechanism regulating these receptors is desensitization of heterotrimeric G protein signaling. Classically, this is mediated by a two- step process in which activated receptors are phosphorylated by G protein-coupled receptor kinases, leading to the binding of a -arrestin (arr) molecule which sterically interdicts further activaion of the G protein. More recently it has become clear that arrs can also serve as multifunctional adaptors which act as signal transducers in their own right. Moreover, many receptors ligands can be found which disproportionately activate either G protein- or arr-mediated signaling - i.e. biased ligands which may possess greater specificity of action and fewer side effects. One such ligand for the angiotensin II type 1 receptor (AT1R) is now in clinical trials for decompensated congestive heart failure. Accordingly this proposal has three closely linked aims which involve developing a molecular- and atomic-level understanding of how such arr-mediated signaling is generated using as model systems two receptors of great cardiovascular significance, the 2- adrenergic receptor and the AT1R. 1) To determine the molecular mechanisms underlying biased agonism at the AT1R utilizing a remarkable panel of both G- and arr-biased AT1R peptide ligands which we have previously characterized. The dynamic biophysical techniques of electron paramagnetic resonance and hydrogen deuterium exchange mass spectrometry will be utilized to reveal the nature of the biased receptor conformations. 2) To determine the molecular mechanisms underlying 7TMR-arr interactions by applying these same techniques to complexes of these two receptors with arr, obtaining information on both receptor and arr conformations. 3) To crystallize and determine the structures of these 7TMR-arr complexes. The insights which we will generate have the potential to guide the design of powerful new cardiovascular drugs and will further our understanding of the conserved signaling mechanisms of the greater 7TMR family.